The performance of Micro Electro Mechanical Systems (MEMS) devices is significantly impacted by environmental effects, most typically thermal fluctuations/variations and vibrations. For example, in the field of Quartz resonators (oscillators, monolithic filters, etc.) the center frequency must be maintained over a −40 C and 80 C temperature range to within parts per million (ppm) and in some cases parts per billion (ppb) (See for example: J. R. Vig, “Quartz Crystal Resonators and Oscillators For Frequency Control and Timing Applications—A Tutorial” January 2007). This requirement adds the need for a compensation circuit and/or ovenization, significantly increasing the thermal and power budget of the devices. Phase noise requirements of such devices require vibration sensitivity as low as ppb per g of acceleration (see for example R. Filler, “The Acceleration Sensitivity of Quartz Crystal Oscillators: A Review”, IEEE Transactions of Ultrasonics, Ferroelectrics, and Frequency Control, Vo. 35 No. 3, 1998).
Quartz devices are one of many precision mechanical sensors that suffer from environmental effects. Another example is mechanical gyroscopes. A gyroscope that has a bias stability of 0.01 deg/hr in a laboratory environment will quickly degrade by 10 to 100× in bias stability when taken into the field. Over the years, at the macro scale, designs for flexible mounting, ovenization, and material selections have matured, albeit with increase in cost, size, weight, and power (cSWAP).
At the micro scale the technology has yet to mature, due to a different set of challenges in fabrication and design. In heterogeneously integrated MEMS devices a functional MEMS device structure, for example fabricated from a material such as Quartz, Aluminum nitride, compound semiconductors, etc., is to be assembled to a substrate that can be fabricated from a different material, typically Si or another electronic substrate (Si, InP, GaN, etc). Connections between the device and the substrate are necessary to provide both mechanical support and electrical contact.
FIG. 1 shows a cross section of a prior art wafer scale heterogeneous wafer bonding integration process. In FIG. 1, a MEMS device 10 having connection pads 12 on one side and having another side attached to a handle structure 14, is aligned with matching connection pads 16 on a substrate 18. Substrate 18 may be provided for carrying integrated circuitry (not shown) or additional discrete components (not shown).
FIG. 2 shows a cross section of a MEMS assembly 20 comprising MEMS device 10 attached to substrate 18 by the bonding of connection pads 12 to connection pads 16. Handle 10 was removed after the bonding of connection pads 12 to connection pads 16. A problem arising with MEMS assembly 20 is that, due to the stiff nature of the bond typically created between connection pads 12 and 16, stress and strain may form with thermal variations, due to thermal expansion mismatch between the materials of MEMS 10 and substrate 18. This can occur both during the fabrication process and/or out in the operating environment. Furthermore, due to the stiff nature of the connection, mechanical vibrations are easily transferred directly from the substrate to the device, thus reducing performance of the device.
A known solution used to address thermal expansion mismatch problems, for example in the manufacturing of commercial quartz resonators, comprises gluing the resonator into the package using a conductive epoxy. However, this process is a slow and manually intensive task that increases time and cost of production.
Vibration insensitivity can be designed into the device structure. However, such designs generally make fabrication and operation of the device more difficult.
Another approach to vibration and temperature variation is to build platforms on which the device sits (See for example: S. W. Yoon, “Vibration Isolation and Shock Protection for MEMS”, Michigan University Thesis, 2009). It is noted however that the design, manufacturing, and operation of such platforms at the micro scale add a significant amount of complexity and cost.
There exist a need for a structure and method allowing to mount a MEMS on a substrate in a fast and economical way, while protecting the MEMS from vibration of the substrate as well as protecting the MEMS and substrate from thermal expansion mismatch between the materials of MEMS and substrate.